Target acquisition

Field centering

The physical centres of the various imaging arrays on the LT are
not necessarily aligned with the optical axis of the telescope. The
LT's normal acquisition procedure is therefore to first place the
telescope's nominal pointing centre on the target (i.e., "blind
pointing") and then issue a "Focal Plane" command which offsets the
centre of the instrument field-of-view onto the requested coordinates.
This process may be subject to an absolute pointing error of around 10
arcseconds. While this is usually adequate for imaging, more precise
acquisition is required for spectroscopy.

Target acquisition for spectroscopy may be achieved in two
ways. Initially, an image of the target field is always obtained using
the LT's work-horse wide-field optical imager (currently IO:O). That
image is then used either to identify the brightest source in
the field and to centre on that target, or to calibrate the
pointing of the telescope by identifying stars in the field and
fitting a world-coordinate system (WCS) to the image. Based on this
WCS-fitting process, the telescope is offset to move the target
position onto the spectrometer (for FRODOspec this is to within 2
arcsec of the centre of the fibre bundle). Two or three images may be
used to fine-tune the telescope position before switching to
spectroscopy mode and beginning the observation.

For more details see the FRODOspec instrument
page. Note that this FINE TUNE acquisition process can in principle
be used for any instrument, not just with the LT's spectrometers.
However, this process increases the overheads on the project (by a few
minutes) and should therefore only be used if telescope pointing at
the &pm;2arcsec level is important.

Cardinal pointing

Due to flexibility problems with the coolant pipes, we were forced
to dispense with the practice of routing them through the cable wrap
in the cassegrain axis rotator. For most observations the rotator is
therefore set to one of the "cardinal" sky angles of 0, 90, 180 or 270
degrees. This means that images will be automatically rotated to one
of these angles, the choice being the one that will give the longest
possible uninterrupted observing time before reaching a mechanical
rotator limit. It is possible that the sky angle will change during a
(very) long series of exposures - but not during an exposure!

Specific
control of field rotation is available for cases where the science
requirements of the programme demand it, but in general we recommend
leaving the rotator on its default automated setting.

Though observations at different epochs may be obtained at different sky
orientations, the default choice of "cardinal" sky angles means that the overlap between
repeated images of the same field is always maximized,
which is important for programmes relying on secondary standards
within the fields. The standard "North at the top" image display orientation is
simple to achieve in an image viewer, using only 90 degree rotations.

Pointing below altitude 40 degrees

Occasionally the telescope does not point well below Alt ~ 40
degrees. This is due to a problem with the mirror support on one of
the three sectors behind the mirror cell. Images can be obtained that
are shifted by 30-40 arcsec. However, this problem does not affect
telescope tracking or guiding, so the image quality itself should not
be affected. If your science requires that a target be positioned on the
same spot on the CCD each time it is observed, acquisition similar to
what is used for spectroscopy can be used (as noted above). Please contact us for
further details.